So…if you’ve got an mp3 or AAC, and you want to have info like the song title, artist, genre, bpm, key, etc show up in your player, you would fill in that info in the file’s ID3 tags.  It’s simply put a standard for digital music metadata.  When you rip music from a CD, most encoders, if using the aforementioned Gracenote information, will convert the CD’s data to ID3 tags.